Systems and methods for determining an absolute position of a motor in an active front steering system of a vehicle

ABSTRACT

Systems and methods for determining an absolute position of a motor of an active front steering system of the vehicle are provided. In particular, the systems and methods accurately determine an absolute position of the motor upon startup of the active front steering system.

TECHNICAL FIELD

This application relates to systems and methods for determining anabsolute position of a motor in an active front steering system of avehicle

BACKGROUND

Front steering systems have been utilized to assist vehicle operators insteering a vehicle. Front steering system generally includes a handwheeloperably coupled through a gear assembly to a rack and pinion assemblythat is further operably coupled to a pair road wheels. The motor isoperably coupled to the gear assembly for variably adjusting a positionof the road wheels relative to a position of the handwheel.

A relative motor position sensor may be operably coupled to the motor togenerate signals indicative of a relative position of the handwheelduring operation of a vehicle. However, after shutdown of a vehicle, avehicle operator may move the handwheel to a different operationalposition. Accordingly, upon startup of the vehicle, a controllerreceiving the signals from the relative position sensor may not be ableto accurately determine an absolute position of the handwheel.

Accordingly, the inventors herein have recognized a need for an improvedfront steering system that can accurately determine an absolute positionof the vehicle handwheel.

SUMMARY

A method for determining an absolute rotational position of a motor inan active front steering system of a vehicle in accordance with anexemplary embodiment is provided. The active front steering system has ahandwheel operably coupled through a gear assembly to an output member.The motor has a rotor operably coupled to the gear assembly foradjusting a position of the output member relative of a position of thehandwheel. The method includes storing an offset value indicative of arevolution number of the rotor in a memory, upon shutdown of the activefront steering system. The method further includes generating a firstsignal from a motor position sensor indicative of the relativerotational position of the rotor upon startup of the active frontsteering system. The method further includes determining a firstrelative rotational position value based on the first signal. The methodfurther includes calculating a first absolute rotational position valueindicative of an absolute rotational position of the rotor by summingthe first relative rotational position value and the offset value.

A system for determining an absolute rotational position of a motor inan active front steering system of a vehicle in accordance with anotherexemplary embodiment is provided. The active front steering system has ahandwheel operably coupled through a gear assembly to an output member.The motor has a rotor operably coupled to the gear assembly foradjusting a position of the output member relative of a position of thehandwheel. The system includes a controller configured to store anoffset value indicative of a revolution number of the rotor in a memory,upon shutdown of the active front steering system. The system furtherincludes a motor position sensor operably communicating with thecontroller. The motor position sensor is configured to generate a firstsignal indicative of the relative rotational position of the rotor uponstartup of the active front steering system. The controller is furtherconfigured to determine a first relative rotational position value basedon the first signal. The controller is further configured to calculate afirst absolute rotational position value indicative of an absoluterotational position of the rotor by summing the first relativerotational position value and the offset value.

A method for determining an absolute rotational position of a motor inan active front steering system of a vehicle in accordance with anotherexemplary embodiment is provided. The active front steering system has ahandwheel operably coupled through a gear assembly to an output member.The motor has a rotor operably coupled to the gear assembly foradjusting a position of an output member relative of a position of thehandwheel. The method includes generating a first signal from a firstposition sensor indicative of an absolute rotational position of thehandwheel. The method further includes generating a second signal from asecond position sensor indicative of a relative rotational position ofthe rotor of the motor. The method further includes generating a thirdsignal from a third position sensor indicative of an absolute rotationalposition of the output member. The method further includes determining afirst absolute rotational position value corresponding to the absoluterotational position of the handwheel based on the first signal. Themethod further includes determining a relative rotational position valuecorresponding to the relative rotational position of the rotor based onthe second signal. The method further includes determining a secondabsolute rotational position value corresponding to the absoluterotational position of the output member based on the third signal. Themethod further includes determining a position offset value associatedwith the rotor based on the first absolute rotational position value andthe second absolute rotational position value. The method furtherincludes determining a third absolute rotational position valuecorresponding to the absolute rotational position of the rotor based onthe relative rotational position value of the rotor and the positionoffset value.

A system for determining an absolute rotational position of a motor inan active front steering system of a vehicle in accordance with anotherexemplary embodiment is provided. The active front steering system has ahandwheel operably coupled through a gear assembly to an output member.The motor has a rotor operably coupled to the gear assembly foradjusting a position of an output member relative of a position of thehandwheel. The system includes a first position sensor configured togenerate a first signal indicative of an absolute rotational position ofthe handwheel. The system further includes a second position sensorconfigured to generate a second signal indicative of a relativerotational position of the rotor of the motor. The system furtherincludes a third position sensor configured to generate a third signalindicative of an absolute rotational position of the output member. Thesystem further includes a controller operably communicating with thefirst, second, and third position sensors. The controller is configuredto determine a first absolute rotational position value corresponding tothe absolute rotational position of the handwheel based on the firstsignal. The controller is further configured to determine a relativerotational position value corresponding to the relative rotationalposition of the rotor based on the second signal. The controller isfurther configured to determine a second absolute rotational positionvalue corresponding to the absolute rotational position of the outputmember based on the third signal. The controller is further configuredto determine a position offset value associated with the rotor based onthe first absolute rotational position value and the second absoluterotational position value. The controller is further configured todetermine a third absolute rotational position value corresponding tothe absolute rotational position of the rotor based on the relativerotational position value of the rotor and the position offset value.

A method for controlling an active front steering system of a vehicle inaccordance with another exemplary embodiment is provided. The activefront steering system has a handwheel operably coupled through a gearassembly to an output member. The motor has a rotor operably coupled tothe gear assembly for adjusting a position of the output member relativeto a position of the handwheel. The method includes determining a firstposition error value associated with the rotor based on a desiredrotational position value and a measured absolute rotational positionvalue associated with the rotor. The method further includes determiningwhen an operator is turning the handwheel. The method further includesincreasing a commanded rotational position value associated with therotor of the motor toward the desired rotational position value when theoperator is turning the handwheel and the first position error value isgreater than the threshold value. The method further includesdetermining a second position error value associated with the rotorbased on the commanded rotational position value and a measured absoluterotational position value associated with the rotor. The method furtherincludes determining a motor command value based on the second positionerror value and a predetermined gain value obtained from a plurality ofgain values. The method further includes applying the motor commandvalue to the motor to move the rotor toward a desired rotationalposition.

A system for controlling an active front steering active front steeringsystem of a vehicle in accordance with another exemplary embodiment isprovided. The active front steering system has a handwheel operablycoupled through a gear assembly to an output member. The motor has arotor operably coupled to the gear assembly for adjusting a position ofan output member relative of a position of the handwheel. The systemincludes a controller configured to determine a first position errorvalue associated with the rotor based on a desired rotational positionvalue and a measured absolute rotational position value associated withthe rotor. The controller is further configured to determine when anoperator is turning the handwheel. The controller is further configuredto increase a commanded rotational position value associated with therotor of the motor toward the desired rotational position value when theoperator is turning the handwheel and the first position error value isgreater than the threshold value. The controller is further configuredto determine a second position error value associated with the rotorbased on the commanded rotational position value and a measured absoluterotational position value associated with the rotor. The controller isfurther configured to determine a motor command value based on thesecond position error value and a predetermined gain value obtained froma plurality of gain values. The system further includes a motor controlcircuit operably communicating with the controller. The motor controlcircuit is configured to apply the motor command value to the motor tomove the rotor toward a desired rotational position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a vehicle having an active front steeringsystem in accordance with an exemplary embodiment;

FIGS. 2-3 are flowcharts of a method for determining an absoluterotational position value of a motor in accordance with anotherexemplary embodiment, utilized in the active front steering system ofFIG. 1;

FIGS. 4-7 are flowcharts of another method for determining an absoluterotational position value of a motor in accordance with anotherexemplary embodiment, utilized in the active front steering system ofFIG. 1;

FIGS. 8-10 are flowcharts of another method for determining an absoluterotational position value of a motor in accordance with anotherexemplary embodiment, utilized in the active front steering system ofFIG. 1; and

FIG. 11-12 are flowcharts of a method for controlling a motor in theactive front steering system of FIG. 1.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, a vehicle 10 having an active front steering system12 is illustrated. The active front steering system 12 is provided toallow an operator to steer the vehicle 10 in a desired direction. Theactive front steering system 12 includes a handwheel 20, an upper shaft22, an intermediate shaft 24, a tension bar 26, a differential assembly28, a pinion 50, a steering rack 52, a rack shaft 54, wheels 56, 58, amotor control circuit 60, a motor 62, a motor locking device 66, ahandwheel position sensor 68, a motor position sensor 70, a pinionposition sensor 72, and a controller 74.

The handwheel 20 is provided to allow operator to turn the upper shaft22 for steering the vehicle 10. The upper shaft 22 is operably coupledto the intermediate shaft 24 which is further operably coupled to thetorsion bar 26. Accordingly, rotation of the handwheel 20 inducesrotation of the upper shaft 22, the intermediate shaft 44, and thetorsion bar 26. The torsion bar 26 is operably coupled to thedifferential assembly 28.

The differential assembly 28 is provided to assist the operator insteering the vehicle 10. The differential assembly 28 includes gears 30,32, 34, 36, 38 and shafts 40, 42. The gear 30 is operably coupled to thetorsion bar 26. The gear 32 is coupled to a first end of the shaft 40.The gear 36 is operably coupled to a second end of the shaft 40.Further, the gear 34 is coupled to the shaft 40 between the gear 32 andthe gear 36. The gear 34 is further operably coupled to a rotor 64 ofthe motor 62. The gear 36 is operably coupled to the gear 38 which isfurther coupled to the shaft 42. During operation, rotation of the gear30 induces rotation of the gears 32, 34, 36, 38 and rotation of theshaft 42.

The pinion 50 is operably coupled to the shaft 42 and is furtheroperably coupled to the steering rack 52. During operation, rotation ofthe pinion 50 induces the steering rack 52 to move the rack shaft 54 formoving the vehicle wheels 56, 58 in a desired direction.

The motor control circuit 60 is provided to generate motor controlsignals for controlling operation of the motor 62, in response tosignals received from the controller 74. The motor 62 is provided torotate a rotor 64 operably coupled to the gear 34 to generate an assisttorque for assisting an operator in moving the wheels 56, 58 in adesired direction. In an alternative embodiment of the motor 62, themotor 62 is configured to not rotate upon de-energization of the motor62.

The locking device 66 is operably coupled to the motor 62 and isprovided to lock a position of the rotor 64 at a desired position, inresponse to a control signal from the controller 74. In particular, thelocking device 66 is utilized to lock the position of the rotor at thedesired position, during shutdown of the active front steering system 12or when an operational fault condition has occurred in the active frontsteering system 12.

The handwheel position sensor 68 is provided to generate a signalindicative of an absolute position of the handwheel 20 which is receivedby the controller 74. The handwheel position sensor 68 may be operablycoupled to the upper shaft 22.

The motor position sensor 70 is provided to generate a signal indicativeof a relative position of the rotor 64 which is received by thecontroller 74. In an exemplary embodiment, the motor position sensor 70generates a signal indicative of a position of the rotor 64 which isaccurate to within one revolution of the rotor 64. Of course, in analternative embodiment, the motor position sensor 70 could generate asignal that is merely indicative of the relative position of the rotor64.

The pinion position sensor 72 is provided to generate a signalindicative of an absolute position of the pinion 50 which is received bythe controller 74. The pinion position sensor 72 may be disposedproximate either the shaft 42 or the pinion 50.

The controller 74 is provided to control operation of the active frontsteering system 12. In particular, the controller 74 is operably coupledto the handwheel position sensor 68, the motor position sensor 70, andthe pinion position sensor 72 and receives position signals from theposition sensors. Further, the controller 74 is configured to generatecontrol signals that the motor control circuit 60 utilizes to controlthe motor 62. Further, the controller 74 is configured to generatecontrol signals to induce the locking device 66 to either lock aposition of the rotor 64 at a predetermined position or to unlock therotor 64 so that the rotor 64 can rotate. The controller 74 includes aCPU (not shown) and a memory for storing executable instructions thatimplement the methods described herein.

Referring to FIGS. 2-3, a method for determining an absolute position ofthe rotor 64 in accordance with an exemplary embodiment will bedescribed.

At step 90, the handwheel position sensor 68 generates a first signalindicative of an absolute rotational position of the handwheel 20.

At step 92, the controller 74 determines an absolute rotational positionvalue (θ_Handwheel_Absolute_New) corresponding to the absoluterotational position of the handwheel 20 based on the first signal.

At step 94, the motor position sensor 70 generates a second signalindicative of a relative rotational position of the rotor 64 of themotor 62.

At step 96, the controller 74 determines a relative rotational positionvalue (θ_Motor_Relative_New) corresponding to the relative rotationalposition of the rotor 64 based on the second signal.

At step 98, the pinion position sensor 72 generates a third signalindicative of an absolute rotational position of a pinion member 50.

At step 100, the controller 74 determines an absolute rotationalposition value (θ_Pinion_Absolute_New) corresponding to the absoluterotational position of the pinion member 50 based on the third signal.

At step 102, the controller 74 determines a position offset value(θ_Motor_Offset) associated with the rotor 64 utilizing the followingequation:

θ_Motor_Offset=(θ_Pinion_Absolute_New−(K1*θ_Handwheel_Absolute_New))/K2; where K1 and K2 are predeterminedconstants.

At step 104, the controller 74 determines an absolute rotationalposition value (θ_Motor_Absolute_New) corresponding to an absoluterotation position of the rotor 64 utilizing the following equation:θ_Motor_Absolute_New=θ_Motor_Relative_New+θ_Motor_Offset.

At step 106, the controller 74 makes a determination as to whetherclosed loop control of the motor 62 can be performed. If the value ofstep 106 equals “yes”, the method advances to step 108. Otherwise, themethod advances to step 110.

At step 108, the controller 74 initiates the system ramp-up algorithmwhich will be explained in greater detail below. After step 108, themethod is exited.

At step 110, the controller 74 removes electrical power from the motor62. After step 110, the method is exited.

Referring to FIGS. 4-7, a method for determining an absolute position ofthe rotor 64 in accordance with another exemplary embodiment will bedescribed. The following method can be utilized with an alternativeembodiment of the active front steering system 12 that utilizes thehandwheel position sensor 68 and the motor position sensor 70, but doesnot utilize the pinion position sensor 72.

At step 120, the controller 74 generates a first signal to engage alocking device 66 to prevent movement of the rotor 64 of the motor 62upon shutdown of the active front steering system 12.

At step 122, the controller 74 stores a rotational position value(θ_Handwheel_Absolute) indicative of an absolute rotational position ofthe handwheel 20 in a memory, upon shutdown of the active front steeringsystem 12.

At step 124, the controller 74 stores a rotational position value(θ_Motor_Absolute) indicative of an absolute rotational position of therotor 64 within a single turn of the rotor 64 in the memory, uponshutdown of the active front steering system 12.

At step 126, the controller 74 stores an offset value (θ_Motor_Offset)indicative of a revolution number of the rotor 64 in the memory, uponshutdown of the active front steering system 12.

At step 128, the handwheel position sensor 68 generates a second signalindicative of an absolute rotational position of the handwheel 20, uponstartup of the active front steering system 12.

At step 130, the controller 74 determines an absolute rotationalposition value (θ_Handwheel_Absolute_New) corresponding to the absoluterotational position of the handwheel 20 based on the second signal.

At step 132, the motor position sensor 70 generates a third signalindicative of the relative rotational position of the rotor 62 within asingle turn of the rotor 62, upon startup of the active front steeringsystem 12.

At step 134, the controller 74 determines a relative rotational positionvalue (θ_Motor_Relative_New) corresponding to the relative rotationalposition of the rotor 62 based on the third signal.

At step 136, the controller 74 retrieves the (θ_Handwheel_Absolute)value from the memory.

At step 138, the controller 74 retrieves the (θ_Motor_Absolute) valuefrom the memory.

At step 140, the controller 74 retrieves the (θ_Motor_Offset) value fromthe memory.

At step 142, the controller 74 determines an absolute rotationalposition value (θ_Motor_Absolute_New) corresponding to an absoluterotational position of the rotor 62 utilizing the following equation:

θ_Motor_Absolute_New=θMotor_Offset+θ_Motor_Relative_New.

At step 144, the controller 74 makes a determination as to whether theθ_Motor_Absolute_New is equal to θ_Motor_Absolute, indicating rotor 62has not moved since last shutdown of active front steering system 12. Ifthe value of step 144 equals “yes”, the method advances to step 160.Otherwise, the method advances to step 146.

At step 146, the controller 74 generates a fourth signal to disengagethe locking device 66 to allow movement of the rotor 62.

At step 148, the controller 74 makes a determination as to whetherclosed loop control of the motor 62 can be performed. If the value ofstep 148 equals “yes”, the method advances to step 150. Otherwise, themethod advances to step 156.

At step 150, the controller 74 generates a fifth signal to induce themotor 62 to move the rotor 64 to a position corresponding to the(θ_Motor_Absolute) value.

At step 152, the controller 74 generates a sixth signal to engage thelocking device 66 to prevent movement of the rotor 64 of the motor 62.

At step 154, the controller 74 removes electrical power from the motor62. After step 154, the method is exited.

Referring again to step 148, when the value of step 148 equals “no”, themethod advances to step 156. At step 156, the controller 74 generates aseventh signal to engage the locking device 66 to prevent movement ofthe rotor 64 of the motor 62.

At step 158, the controller 74 removes electrical power from the motor62. After step 158, the method is exited.

Referring again to step 144, when the value of step 144 equals “yes”,the method advances to step 160. At step 160, the controller 74 makes adetermination as to whether the θ_Handwheel_Absolute_New is equal toθ_Handwheel_Absolute, indicating the handwheel 20 has not moved sincelast shutdown of active front steering system 12.

At step 162, the controller 74 generates an eighth signal to disengagethe locking device 66 to allow movement of the rotor 64.

At step 164, the controller 74 makes a determination as to whetherclosed loop control of the motor 62 can be performed. If the value ofstep 164 equals “yes,” the method advances to step 166. Otherwise, themethod advances to step 168.

At step 166, controller 74 initiates the system ramp-up algorithm whichwill be explained in greater detail below. After step 166, the method isexited.

Referring again to step 164, when the value of step 164 equals “no”, themethod advances to step 168. At step 168, the controller 74 generates aninth signal to engage the locking device 66 to prevent movement of therotor 64 of the motor 62.

At step 170, the controller 74 removes electrical power from the motor62. After step 170, the method is exited.

Referring to FIGS. 8-10, a method for determining an absolute positionof the rotor 64 in accordance with another exemplary embodiment will bedescribed. The following method can be utilized with an alternativeembodiment of the active front steering system 12 that utilizes thehandwheel position sensor 68 and the motor position sensor 70, but doesnot utilize the pinion sensor 72 and the locking device 66.

At step 180, the controller 74 stores a rotational position value(θ_Handwheel_Absolute) indicative of an absolute rotational position ofthe handwheel 20 in a memory, upon shutdown of the active front steeringsystem 12.

At step 182, the controller 74 stores a rotational position value(θ_Motor_Absolute) indicative of an absolute rotational position of therotor 64 within a single turn of the rotor 64 in the memory, uponshutdown of the active front steering system 12.

At step 184, the controller 74 stores an offset value (θ_Motor_Offset)indicative of a revolution number of the rotor 64 in the memory, uponshutdown of the active front steering system 12.

At step 186, the handwheel position sensor 68 generates a first signalindicative of an absolute rotational position of the handwheel 20, uponstartup of the active front steering system 12.

At step 188, the controller 74 determines an absolute rotationalposition value (θ_Handwheel_Absolute_New) corresponding to the absoluterotational position of the handwheel 20 based on the first signal.

At step 190, the motor position sensor 70 generates a second signalindicative of the relative rotational position of the rotor 64 within asingle turn of the rotor 64, upon startup of the active front steeringsystem 12.

At step 192, the controller 74 determines a relative rotational positionvalue (θ_Motor_Relative_New) corresponding to the relative rotationalposition of the rotor 64 based on the second signal.

At step 194, the controller 74 retrieves the (θ_Handwheel_Absolute)value from the memory.

At step 196, the controller 74 retrieves the (θ_Motor_Absolute) valuefrom the memory.

At step 198, the controller 74 retrieves the (θ_Motor_Offset) value fromthe memory.

At step 200, the controller 74 determines an absolute rotationalposition value (θ_Motor_Absolute_New) corresponding to an absoluterotational position of the rotor 64 utilizing the following equation:

θ_Motor_Absolute_New=θ_Motor_Offset+θ_Motor_Relative_New.

At step 202, the controller 74 makes a determination as to whether theθ_Handwheel_Absolute is equal to θ_Handwheel_Absolute_New, indicatingthe handwheel 20 has not moved since last shutdown of active frontsteering system 12. If the value of step 202 equals “yes”, the method isexited. Otherwise, the method advances to step 204.

At step 204, the controller 74 makes a determination as to whetherclosed loop control of the motor 62 can be performed. If the value ofstep 204 equals “yes,” the method advances to step 206. Otherwise, themethod advances to step 208.

At step 206, the controller 74 initiates the system ramp-up algorithmwhich will be explained in greater detail below. After step 206, themethod is exited.

At step 208, the controller 74 removes electrical power from the motor62. After step 208, the method is exited.

Referring to FIGS. 11-12, a system ramp-up method or algorithm forcontrolling the active front steering system 12 in accordance withanother exemplary embodiment is provided.

At step 210, the controller 74 determines a position error value(θ_Motor_Error) associated with the rotor 64 based on a desiredrotational position value (θ_Motor_Desired) and a measured absoluterotational position value (θ_Motor_Absolute_New) associated with therotor 64.

At step 212, the handwheel position sensor 68 generates a signalindicative of an absolute position of the handwheel 20.

At step 214, the controller 74 determines a handwheel rotationalvelocity of the utilizing the signal from the handwheel position sensor68

At step 216, the controller 74 makes a determination as to whetherθ_Motor_Error is less then a threshold value. If the value of step 216equals “yes”, the method is exited. Otherwise, the method advances tostep 218.

At step 218, the controller 74 makes a determination as to whether thehandwheel rotational velocity is greater than a predetermined velocity.If the value of step 218 equals “yes”, the method advances to step 220.Otherwise, the method returns to step 218.

At step 220, the controller 74 increases a commanded rotational positionvalue (θ_Motor_Command) associated with the rotor 64 of the motor 62toward a desired rotational position value (θ_Motor_Desired) when theoperator is turning the handwheel 20, utilizing the following equation:

θ_Motor_Command=[θ_Motor_Absolute_New*[1−U(t)]+θ_Motor_Desired*U(t)];where U(t) is a ramp function with a calibratable slope.

At step 222, the controller 74 determines a position error value(θ_Motor_Servo_Error) associated with the rotor 64 utilizing thefollowing equation:θ_Motor_Servo_Error=θ_Motor_Command−θ_Motor_Absolute_New.

At step 224, the controller 74 determines a motor command value(V_Motor_Command) utilizing the following equation:V_Motor_Command=θ_Motor_Servo_Error*G1, where G1 is selected from atable having a plurality of gain values.

At step 226, the controller 74 applies the motor command value(V_Motor_Command) to move the rotor 64 toward a desired rotationalposition.

At step 228, the controller 74 makes a determination as to whether theθ_Motor_Servo_Error is less then a threshold motor servo error. If thevalue of step 228 equals “yes”, the method is exited. Otherwise, themethod returns to step 228.

The systems and methods for determining an absolute position of motorand an active front steering system provide a substantial advantage overother systems and methods. In particular, the systems and methodsprovide a technical effect of accurately determining a rotationalposition of the motor after shutdown of the active front steering systemand subsequent activation of the active front steering system.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the presentapplication.

1. A method for determining an absolute rotational position of a motorin an active front steering system of a vehicle, the active frontsteering system having a handwheel operably coupled through a gearassembly to an output member, the motor having a rotor operably coupledto the gear assembly for adjusting a position of the output memberrelative of a position of the handwheel, the method comprising: storingan offset value indicative of a revolution number of the rotor in amemory, upon shutdown of the active front steering system; generating afirst signal from a motor position sensor indicative of the relativerotational position of the rotor upon startup of the active frontsteering system; determining a first relative rotational position valuebased on the first signal; and calculating a first absolute rotationalposition value indicative of an absolute rotational position of therotor by summing the first relative rotational position value and theoffset value.
 2. The method of claim 1, further comprising: storing asecond absolute rotational position value indicative of an absoluterotational position of the handwheel upon shutdown of the active frontsteering system; generating a second signal from a handwheel positionsensor indicative of an absolute rotational position of the handwheelupon startup of the active front steering system; determining a thirdabsolute rotational position value based on the second signal; andcomparing the second absolute rotational position value to the thirdabsolute rotational position value to determine whether the handwheelhas moved between the shutdown of the active front steering system andthe startup of the active front steering system.
 3. The method of claim2, further comprising: determining whether the motor is controllable;and removing electrical power from the motor when both the handwheel hasmoved between the shutdown of the active front steering system and thestartup of the active front steering system and the motor is notcontrollable.
 4. The method of claim 2, further comprising: determiningwhether the motor is controllable; and adjusting a position of the rotortoward a desired absolute position when both the handwheel has movedbetween the shutdown of the active front steering system and the startupof the active front steering system and the motor is controllable. 5.The method of claim 1, further comprising: engaging a motor lockingdevice upon shutdown of the active front steering system to prevent therotor from rotating.
 6. The method of claim 1, further comprising:storing a second absolute rotational position value indicative of anabsolute rotational position of the rotor upon shutdown of the activefront steering system; and comparing the first absolute rotationalposition value to the second absolute rotational position value todetermine whether the rotor has moved between the shutdown of the activefront steering system and the startup of the active front steeringsystem.
 7. The method of claim 6, further comprising: disengaging themotor locking device to allow the rotor to rotate when the rotor hasmoved between the shutdown of the active front steering system and thestartup of the active front steering system; determining whether themotor is controllable; removing electrical power from the motor when themotor is not controllable.
 8. The method of claim 6, further comprising:storing a third absolute rotational position value indicative of anabsolute rotational position of the handwheel upon shutdown of theactive front steering system; generating a second signal from ahandwheel position sensor indicative of an absolute rotational positionof the handwheel upon startup of the active front steering system;determining a fourth absolute rotational position value based on thesecond signal; and comparing the third absolute rotational positionvalue to the fourth absolute rotational position value to determinewhether the handwheel has moved between the shutdown of the active frontsteering system and the startup of the active front steering system. 9.The method of claim 8, further comprising: determining whether the motoris controllable; and removing electrical power from the motor when boththe handwheel has moved between the shutdown of the active frontsteering system and the startup of the active front steering system andthe motor is not controllable.
 10. The method of claim 8, furthercomprising: determining whether the motor is controllable; and adjustinga position of the rotor toward a desired absolute position when both thehandwheel has moved between the shutdown of the active front steeringsystem and the startup of the active front steering system and the motoris controllable.
 11. A system for determining an absolute rotationalposition of a motor in an active front steering system of a vehicle, theactive front steering system having a handwheel operably coupled througha gear assembly to an output member, the motor having a rotor operablycoupled to the gear assembly for adjusting a position of the outputmember relative of a position of the handwheel, the system comprising: acontroller configured to store an offset value indicative of arevolution number of the rotor in a memory, upon shutdown of the activefront steering system; a motor position sensor operably communicatingwith the controller, the motor position sensor configured to generate afirst signal indicative of the relative rotational position of the rotorupon startup of the active front steering system; the controller furtherconfigured to determine a first relative rotational position value basedon the first signal; and the controller further configured to calculatea first absolute rotational position value indicative of an absoluterotational position of the rotor by summing the first relativerotational position value and the offset value.
 12. A method fordetermining an absolute rotational position of a motor in an activefront steering system of a vehicle, the active front steering systemhaving a handwheel operably coupled through a gear assembly to an outputmember, the motor having a rotor operably coupled to the gear assemblyfor adjusting a position of an output member relative of a position ofthe handwheel, the method comprising: generating a first signal from afirst position sensor indicative of an absolute rotational position ofthe handwheel; generating a second signal from a second position sensorindicative of a relative rotational position of the rotor of the motor;generating a third signal from a third position sensor indicative of anabsolute rotational position of the output member; determining a firstabsolute rotational position value corresponding to the absoluterotational position of the handwheel based on the first signal;determining a relative rotational position value corresponding to therelative rotational position of the rotor based on the second signal;determining a second absolute rotational position value corresponding tothe absolute rotational position of the output member based on the thirdsignal; determining a position offset value associated with the rotorbased on the first absolute rotational position value and the secondabsolute rotational position value; and determining a third absoluterotational position value corresponding to the absolute rotationalposition of the rotor based on the relative rotational position value ofthe rotor and the position offset value.
 13. A system for determining anabsolute rotational position of a motor in an active front steeringsystem of a vehicle, the active front steering system having a handwheeloperably coupled through a gear assembly to an output member, the motorhaving a rotor operably coupled to the gear assembly for adjusting aposition of an output member relative of a position of the handwheel,the system comprising: a first position sensor configured to generate afirst signal indicative of an absolute rotational position of thehandwheel; a second position sensor configured to generate a secondsignal indicative of a relative rotational position of the rotor of themotor; a third position sensor configured to generate a third signalindicative of an absolute rotational position of the output member; anda controller operably communicating with the first, second, and thirdposition sensors, the controller configured to determine a firstabsolute rotational position value corresponding to the absoluterotational position of the handwheel based on the first signal, thecontroller further configured to determine a relative rotationalposition value corresponding to the relative rotational position of therotor based on the second signal, the controller further configured todetermine a second absolute rotational position value corresponding tothe absolute rotational position of the output member based on the thirdsignal, the controller further configured to determine a position offsetvalue associated with the rotor based on the first absolute rotationalposition value and the second absolute rotational position value, thecontroller further configured to determine a third absolute rotationalposition value corresponding to the absolute rotational position of therotor based on the relative rotational position value of the rotor andthe position offset value.
 14. The system of claim 13, wherein theoutput member comprises at least one of a pinion gear and a vehiclewheel.
 15. A method for controlling an active front steering system of avehicle, the active front steering system having a handwheel operablycoupled through a gear assembly to an output member, the motor having arotor operably coupled to the gear assembly for adjusting a position ofthe output member relative to a position of the handwheel, the methodcomprising: determining a first position error value associated with therotor based on a desired rotational position value and a measuredabsolute rotational position value associated with the rotor;determining when an operator is turning the handwheel; increasing acommanded rotational position value associated with the rotor of themotor toward the desired rotational position value when the operator isturning the handwheel and the first position error value is greater thanthe threshold value; determining a second position error valueassociated with the rotor based on the commanded rotational positionvalue and a measured absolute rotational position value associated withthe rotor; determining a motor command value based on the secondposition error value and a predetermined gain value obtained from aplurality of gain values; and applying the motor command value to themotor to move the rotor toward a desired rotational position.
 16. Themethod of claim 15, further comprising determining the desiredrotational position value based on a handwheel position and a vehiclespeed.
 17. The method of claim 15, wherein determining when the operatoris turning the handwheel comprises: generating a plurality of signals inwhich each signal is indicative of a position of the handwheel; anddetermining a rotational velocity of the handwheel utilizing theplurality of signals; and determining when the velocity of the handwheelis greater than a predetermined velocity which is indicative of theoperator turning the handwheel.
 18. The method of claim 15, whereinincreasing the commanded rotational position value associated with therotor of the motor toward the desired rotational position valuecomprises gradually increasing the commanded rotational position valueutilizing a ramping function.
 19. The method of claim 15, wherein themotor command value is determined by multiplying the second positionerror value by the predetermined gain value.
 20. A system forcontrolling an active front steering active front steering system of avehicle, the active front steering system having a handwheel operablycoupled through a gear assembly to an output member, the motor having arotor operably coupled to the gear assembly for adjusting a position ofan output member relative of a position of the handwheel, the systemcomprising: a controller configured to determine a first position errorvalue associated with the rotor based on a desired rotational positionvalue and a measured absolute rotational position value associated withthe rotor, the controller further configured to determine when anoperator is turning the handwheel, the controller further configured toincrease a commanded rotational position value associated with the rotorof the motor toward the desired rotational position value when theoperator is turning the handwheel and the first position error value isgreater than the threshold value, the controller further configured todetermine a second position error value associated with the rotor basedon the commanded rotational position value and a measured absoluterotational position value associated with the rotor, the controllerfurther configured to determine a motor command value based on thesecond position error value and a predetermined gain value obtained froma plurality of gain values; and a motor control circuit operablycommunicating with the controller, the motor control circuit configuredto apply the motor command value to the motor to move the rotor toward adesired rotational position.